The present invention relates to a process and equipment for the production of high-purity polycrystalline silicon in rod form for semiconductor applications. The polycrystalline silicon is used as the raw material in the fabrication of single crystal silicon for semiconductors by the CZ (Czochralski) method or the FZ (float zone) method.
The most common method of producing polycrystalline silicon, which is a raw material used for the production of single crystal silicon for semiconductors, has been to deposit silicon on starter filaments by thermal decomposition of a chloride-type silane, such as trichlorosilane, so as to produce silicon rods. Such a process is described in U.S. Pat. No. 4,724,160 of Arvidson et al. Japanese Patent Laid-Open No. 56-105622 discloses a reactor structure using a chloride-type silane in which a large number of electrodes are arranged on a circular plate and a large number of silicon starter filaments are arranged in a reverse-U-shaped or a square-reverse-U-shaped form.
Another method involves the production of polycrystalline silicon from monosilane gas, which is another common starting material. Silicon starter filaments are heated inside the reactor. At a temperature of several hundred degrees or more, monosilane gas decomposes and deposits on heated filaments to form larger-diameter silicon rods. The rods may be thermally insulated from each other so as to prevent vapor-phase temperature rise and as to eliminate thermal influences from the adjacent heated silicon rods, thereby obtaining uniform silicon deposition.
Polycrystalline silicon, in the form of rods or chunks obtained by crushing rods, is being widely used in the production of single crystal silicon by the CZ or FZ method. A high purity level and competitive cost are particularly required of polycrystalline silicon rods for semiconductor applications.
In both of the silicon deposition systems described above, a power supply is used to pass current through the rods for such heating rods. The existing power supplies for the thermal decomposition furnaces, such as that shown in U.S. Pat. Nos. 4,147,814 and 4,150,168 of Yatsurugi et al., and U.S. Pat. No. 4,805,556 of Hagan et al., and U.S. Pat. No. 5,478,396 of Keck et al., produce up to 2000 amps of current at 60 Hz. Power supplied at this low 60 Hz frequency (or the European standard power frequency of 50 Hz), allows the heating current to flow throughout the rod's cross-section during thermal decomposition.
When power is supplied at 60 Hz, the current migrates toward the centers of the rods. The center of a rod becomes progressively hotter, relative to the surrounding outer region of the rod, since the center is thermally insulated by the outer region or "skin" of the rod. Heating at the center causes electrical resistance to decrease at the center, since r=f(1/T). The lower resistance causes even more current to flow through the center, which creates more heat. Since a majority of the current flows through the center of the rod when operating at 60 Hz, the center of a rod becomes considerably hotter than the skin portion of the rod. This uneven temperature profile in turn creates internal thermal stresses when the rods cool down following growth, with the resulting rods being brittle and subject to breakage. In particular, when the power is turned off, the surface of the rod cools and contracts quickly to a given depth. The cooled surface acts as an insulative layer to the rod interior. Because of the insulative effect, the center of the rod cools at a much slower rate. The effect of this is that the center material has a lower specific volume, at the termination of cooling, due to the longer relaxation time. (The slower the cooling rate the longer the atoms have to arrange themselves. Thus, the interior of the rod should have a slightly higher density than the exterior.) This has the effect of radially pulling the surface into a compression state while the inner part of the rod is in a state of tension due to the smaller specific volume. It is easy to picture that, as the radius approaches zero, the stresses increase due to cooling rates that are progressively slower as ore approaches the center of the rod. Due to the stresses, to obtain commercially acceptable yields, rods produced with 60 Hz power supplies have been limited to a maximum diameter of about 150 mm.